The small Indian mongoose, Herpestes auropunctalus
(Carnivora: Viverridae), and the roof rat, Rattus rattus, and the Polynesian
rat, Rattus exulans (both Rodentia: Muridae), are abundant in Waipio Valley,
island of Hawaii. Two other murid rodents, the house· mouse, Mus musculus,
and the Norway rat, Rattus norvegicus, are sporadic or rare in occurrence. As
carriers of serotypes of the bacterium Leptospira interrogans (Spirochaetales:
Treponemataceae), which is transmissible to humans, this assemblage of
introduced mammals is of public health significance, for numerous cases of
leptospirosis, or Weil's disease, have been traced to the valley. Population
density of the mongoose was estimated at 2.3 per acre; for rats, it fluctuated
seasonally from I to 11 per acre. The serotypes icterohemorrhagiae and sejroe
were found in the mongoose in a 40:60 ratio by the kidney culture method.
Combined kidney culture and serological tests on 180 mongooses showed a
high of 34 percent overall infection in winter and a summer low of 9.4 percent.
Of 33 house mice tested by culture only, ballum was isolated from 21 and
icterohemorrhagiae from two. One isolation of icterohemorrhagiae was made
from four Norway rats examined. For 126 roof rats tested by serology and
kidney culture, 68 percent of adults and 26 percent of young were infected; and
for 175 Polynesian rats, 34 percent of adults and 26 percent of young were
infected. The Polynesian rat demonstrated a lesser persistence of the serum
titer phase of the disease than did the roof rat. Icterohemorrhagiae made up 95
percent and ballum the remaining 5 percent of infections in the roof rat. For
the Polynesian rat the ratio was 75: 25. Free-ranging rats under observation
for as long as 8 months acquired or lost infections, as determined by repeated
serological tests. The wet subtropical climate of Waipio Valley supports
conditions for transmission of leptospirosis among small mammals, and
possibly to humans, even in times of drought. No prominent differences were
observed in the infection rates in the lower valley at 30 ft above sea level and
1.7 miles inland at 120 ft. In the forested watershed of the valley rim at 3000 ft,
conditions of infection by species of host and by serotype of L. interrogans
matched elosely those found on the valley floor. Tests of 152 water samples
from streams, ponds, and taro patches resulted in isolations only of saprophytic
leptospires, although temperatures, salinities, and pH concentrations
appeared to be favorable for the support of pathogenic forms.